The present invention relates to a syntax converting apparatus to perform conversion between an abstract syntax and a transfer syntax in a presentation layer, defined by an Open Systems Interconnection (OSI), an international standard for the interconnection of systems.
The presentation layer of OSI is a layer to alter the set of sending/receiving codes between application processes, data compression and the cryptographic form of representation. In this layer, the syntax is determined which describes application processes themselves and an exchange is achieved with a syntax to be transferred.
The functional module, which performs actual communication of information related to an application layer, is known as the application entity. The module that performs actual communication of information related to a presentation layer is known as the presentation entity. The form of representation of the data given and received between the application entity and presentation entity is called an abstract syntax, and the form of the data transferred between presentation entities is called, a transfer syntax. The abstract syntax is intended to define, in a standardized manner, the semantics of data represented in an internal code, uniquely specified for an actual computer system or the like. Some techniques for representing the abstract syntax and the transfer syntax, are the ISO (International Organization for Standardization) 8824 ASN.1 (Abstract Syntax Notation One) and the ISO 8825 ASN.1 Basic Encoding Rules.
The method used in related art of decomposing a data string encoded in the presentation layer is to indiscriminately take out the contents' octets defined by the ASN.1 encoding rules. By this method, all of the items are decomposed in the presentation layer, and handed over to the application layer.
Suppose that, in such a syntax converting apparatus, it has become necessary to separately decompose presentation protocol data units (PDUs), which are separately defined by Standard x and Standard y. Then, the xPDU defined by Standard x is decomposed by Standard x first, and the yPDU defined by Standard y is decomposed by Standard y later. However, if the identifier octet "31"H indicating the SET type at the top of the yPDU is absent, the yPDU cannot be decomposed by Standard y. Therefore, the PDUs separately defined by Standard x and Standard y are obliged to be decomposed together in the presentation layer. This results in the disadvantage that, along with an increase in standards in the application layer, decomposing all the PDUs in the presentation layer invites increases in abstract syntaxes to be managed (provisions on the PDU data structure) and items to be interfaced with an upper layer.